1. Field of the Invention
The present invention relates to an optical transmitter, and in particular to a transmitter suitable for use in the transmission and distribution of analogue optical signals modulated at RF or microwave frequencies.
2. Description of Related Art
The low losses and EMI immunity associated with optical fibres, makes their use an attractive proposition, for example, in the distribution of signals to remote transmitter sites in a cellular radio system. The optical transmitters used hitherto with such optical fibre links have either been in the form of directly modulated laser diodes, or have comprised continuous wave lasers coupled with a separate electro-optic modulator such as a Mach-Zehnder intensity modulator or an electroabsorption modulator. Such optical transmitters suffer however, from a dynamic range which is significantly inferior to that of the electronic devices commonly used in cellular base stations. This has restricted the use of optical analogue links.
For a given received optical power, the dynamic range of analogue opticalfibre links is limited primarily by the linearity of the electrical to optical transfer characteristic of the optical transmitter. It has previously been proposed to use schemes such as electrical pre-distortion or optical feed-forward linearisation to increase the overall system dynamic range. It would be desirable however to improve the intrinsic source linearity. In the context of a source using a continuous wave laser followed by a Mach-Zehnder modulator, it has been proposed to use two modulators in cascade in order to improve linearity [Betts. G.E. IEEE Trans. Microwave Theory & Techniques, vol 42 no. 12 pp 2642-2649]. By feeding an RF drive signal to both modulators in a prescribed ratio, the second modulator corrects for the distortion of the first.